1. Field of the Invention
The present invention relates to a method of reducing the NO.sub.x content in combustion gases by introducing secondary fuel, via a carrier medium, into the combustion chamber at a location disposed between the supply of fuel and air for the primary combustion, and the supply of secondary air for the complete combustion.
2. Description of the Prior Art
It is already known to reduce the NO.sub.x content of the combustion gases within the combustion chamber (furnace or boiler) by carrying out the primary combustion in a reducing atmosphere with an insufficient quantity of air, whereby a portion of the air required for complete combustion being supplied downstream of the primary combustion zone. It is furthermore known, for the purpose of reducing the NO.sub.x content, to supply secondary fuel, by means of a gaseous carrier medium, after the primary combustion zone and prior to the supply of the residual air required for the complete combustion; the flue gas of the primary combustion serves as the carrier medium.
It has become evident that the introduction of secondary fuel with a gaseous carrier, such as air or combustion gas, for reducing the NO.sub.x content, causes difficulties, particularly in large combustion chambers. With a gaseous carrier, the mixture of secondary fuel and carrier does not penetrate deep enough into the combustion chamber. The thorough mixing which is absolutely necessary is therefore achieved only after relatively long mixing times. Furthermore, the distribution and control of the supply of fuel causes difficulties. Generally, a good thorough mixing can be achieved only with a series of nozzles, with the diameter and speed of the nozzles having to be optimized. However, the arrangement and number of such nozzles is structurally very complicated and expensive. Furthermore, when individual introduction or injection locations are not operative, significant impairment of the operation results. In the extreme case, the entire unit must be shut down. In addition, when a large capacity unit is being operated, the type of secondary fuel introduction required for an optimum reduction of the NO.sub.x content has to be adjusted as a function of load conditions. This variation of the type of introduction as a function of the load in turn requires considerable structrual expense. For the aforementioned reasons, the previously described reduction of the NO.sub.x content via secondary fuel was utilized only in test installations, but not in large-scale units, i.e. on the order of magnitude of 1000 to 2000 metric tons of steam per hour.
An object of the present invention is to provide a method with which, in a straight forward manner, the NO.sub.x content in the combustion gases of large-capacity combustion units, especially steam generators, is effectively reduced.